1. Field of the Invention
The present invention relates to a method for forming a phase grating. In particular, the present invention relates to a method to manufacture the rings of the phase grating to have different height by selective etching.
2. Description of the Prior Art
Grating is an optical element made of periodical fine patterns. The patterns constituting the grating may be the change of transmittance, thickness or index of refraction. Among which, the grating made of change of thickness or index of refraction modulates the phase of the light to have diffraction. It is therefore called “phase grating”.
The substrate for the grating is usually glass or plastics. The transmittance, thickness or index of refraction of the surface is then somehow changed. For example, fine patterns are engraved on the glass substrate by mechanical methods to obtain a reflective grating. Besides, most of the gratings made of plastics are thickness modulation type. First, a mother mold made of metal material is produced then the products are replicated therefrom. The problems of this method are, for example, difficult to control the cross section of the patterns and low yield.
For instance, the conventional Fresnel Lens is known to have the ability to focus without the disadvantages of traditional glass lens, such as heavy weight and bulky size. The Fresnel Lens per se may be composed of sheets made from polymeric materials with its surface engraved with multiple concentric circles. The angles and thickness of each circle is different so the Fresnel Lens is able to focus light.
First, U.S. Pat. No. 5,132,843 discloses a grating lens. Such grating lens is composed of a plurality of concentric circles formed on the surface. However, U.S. Pat. No. 5,132,843 does not disclose the substrate is made of what material or a plurality of concentric circles is formed by which methods on the substrate. Besides, U.S. Pat. No. 6,570,145 discloses another phase grating imaging sensor and the method for making the same. U.S. Pat. No. 6,570,145 discloses that a plurality of concentric circles is formed on a metal material, such as Cr, to obtain the phase grating imaging sensor. The aforesaid phase gratings are Fresnel Lens. The lens of such Fresnel Lens type phase gratings are of the same depth and disposed on the same surface. The focus capability of such planar gratings is not optimal. Accordingly, a phase grating of better focus capability as well as a method which is compatible with the current semiconductor process techniques are needed to overcome the problems which the Fresnel-Lens type phase grating encounters in the semiconductor field.